Posts Tagged ‘frontier molecular orbital theory’

In organic chemistry class, one learns that elimination reactions involve the cleavage of a σ bond and formation of a π bond. A nucleophilic pair of electrons (either from another bond or a lone pair) heads into a new π bond as a leaving group departs. This process is called β-elimination because the bond β to the nucleophilic pair of electrons breaks. Transition metal complexes can participate in their own version of β-elimination, and metal alkyl complexes famously do so. Almost by definition, metal alkyls contain a nucleophilic bond—the M–C bond! This bond can be so polarized toward carbon, in fact, that it can promote the elimination of some of the world’s worst leaving groups, like –H and –CH3. Unlike the organic case, however, the leaving group is not lost completely in organometallic β-eliminations. As the metal donates electrons, it receives electrons from the departing leaving group. When the reaction is complete, the metal has picked up a new π-bound ligand and exchanged one X-type ligand for another.

Comparing organic and organometallic β-eliminations. A nucleophilic bond or lone pair promotes loss or migration of a leaving group.

In this post, we’ll flesh out the mechanism of β-elimination reactions by looking at the conditions required for their occurrence and their reactivity trends. Many of the trends associated with β-eliminations are the opposite of analogous trends in 1,2-insertion reactions. A future post will address other types of elimination reactions.

β-Hydride Elimination

The most famous and ubiquitous type of β-elimination is β-hydride elimination, which involves the formation of a π bond and an M–H bond. Metal alkyls that contain β-hydrogens experience rapid elimination of these hydrogens, provided a few other conditions are met. Read the rest of this entry »

Our romp through the common dative ligands continues with the N-heterocyclic carbenes (NHCs). Although we’ll tackle carbenes in general in another post, NHCs deserve their own nod due to their unique structure, properties, and steric tunability. Unlike most metal carbenes, NHCs are typically unreactive when coordinated to a metal (with some exceptions). Like phosphines, they are commonly used to modulate the steric and electronic properties of metal complexes. In fact, the similarities between NHCs and phosphines are notable. Overall, few ligands are as effective as NHCs at ramping up the electron density on a metal center while remaining innocent.

General Properties

Free NHCs contain carbon in the rarely encountered +2 oxidation state. In general, we can classify carbenes according to the nature of the two non-bonding electrons—if they are spin paired (one up and one down), the carbene is called a singlet; if their spins are parallel, we call the carbene a triplet. Whether a carbene is in the singlet or triplet state depends primarily on the difference in energy between its frontier orbitals—when the FMOs are close in energy, single occupation of each FMO (the triplet state) is likely. As the energy difference increases, the singlet state becomes more likely because the higher-energy FMO is less likely to be occupied.

FMO pictures for singlet and triplet carbenes.

How do we tinker with the FMO energies, you ask? The nature of the R groups is key. When R is electron-donating, the energy of the LUMO is raised through a fairly straightforward n → 2pz orbital interaction. An analogous interaction is responsible for the stability of carbocations adjacent to lone-pair-bearing heteroatoms (such as the oxocarbenium ion). Thanks to this orbital interaction, electron-donating groups stabilize the singlet state…and NHCs are no exception! The figure below depicts only one of two possible n → 2pz interactions in free N-heterocyclic carbenes. The LUMOs of free NHCs are quite high in energy, relative to other kinds of carbenes. Read the rest of this entry »

In this post, we’ll begin to explore the molecular orbital theory of organometallic complexes. Some background in molecular orbital theory will be beneficial; an understanding of organic frontier molecular orbital theory is particularly helpful. Check out Fukui’s Nobel Prize lecture for an introduction to FMO theory. The theories described here try to address how the approach of ligands to a transition metal center modifies the electronics of the metal and ligands. The last post on geometry touched on these ideas a little, but we’ll dig a little deeper here. Notably, we need to address the often forgotten influence of the metal on the ligands—how might a metal modify the reactivity of organic ligands?

Ligand Field Theory

The ligand field theory (LFT) fleshes out the ideas of crystal field theory with molecular orbital theory concepts. It provides a method for understanding M–L bonding and antibonding orbitals; however, it has been strongly disputed by computational studies in favor of valence bond models that incorporate hypervalency. Still, LFT provides a more complete picture of complex bonding than crystal field theory, so we’ll discuss it here. Furthermore, the portions of LFT under dispute have nothing to do with CFT, so “no harm no foul.” Let’s take a look at the molecular orbitals of a hypothetical octahedral MLn complex to begin hashing out LFT.

The M–L bonding molecular orbitals mostly have ligand character, while the antibonding orbitals mostly reside on the metal.